1. Field of the Invention
This invention relates to adhesive compositions, particularly co-extrudable adhesives, suitable for composite structures, such as those having both barrier and structural layers. The adhesive compositions are blends of acid-grafted metallocene catalyzed polyethylenes and copolymers of ethylene and vinyl acetate or alkyl acrylates or other ethylenically unsaturated esters or their derivatives.
2. Discussion of Related Art
Co-extrudable adhesives based on blends of various polyethylenes which also contain an acid-grafted polyolefin to aid in adhesion to polar layers are well known.
U.S. Pat. No. 3,868,433 (Bartz et al) discloses polyolefins generally, graft-modified with acids, and which may also contain elastomers, for use as hot-melt adhesives.
U.S. Pat. No. 4,684,576 (Tabor et al) discloses adhesive blends based on acid-grafted high density polyethylene, and linear low density polyethylene of density 0.88 to 0.935.
U.S. Pat. No. 4,452,942 teaches blends of maleic anhydride grafted linear low density polyethylene and EVA, EEA or EMA.
U.S. Pat. No. 4,230,830 discloses blends of grafted HDPE or EPDM in EVA, EMA or E/IBA.
U.S. Ser. No. 08/591,330 relates to adhesive blends comprising grafted metallocene resins and conventional polyethylenes.
Known adhesives containing various polyethylenes and acid-grafted polyethylenes employ polyethylenes which are conventional-linear polyethylenes such as high-density polyethylene homopolymer (conventional-HDPE), and linear low density polyethylene copolymer (conventional-LLDPE), as well as low density polyethylene (high pressure, free-radical or LDPE).
In recent years, polyethylenes have been developed which are made using `single-site` or `metallocene` catalysts. These polyethylenes are dramatically more uniform in various composition related respects. They compare with conventional-HDPE and conventional-LLDPE in that they are essentially linear, containing either no or only a modest amount of long-chain branching, unlike free-radical LDPE which contains large amounts of long-chain branching. In addition, other than the catalysts employed, they can be prepared in ways similar to conventional-HDPE and conventional-LLDPE. They may contain an alpha-olefin comonomer which provides short-chain branching as in conventional-LLDPE.
The catalysts provide uniformity in various ways. The molecular weight distribution is narrow compared with that of conventional-HDPE and conventional-LLDPE. Furthermore, in alpha-olefin copolymers, the comonomer is introduced in a far more uniform way, both along any given chain and from chain to chain, so that the so-called short-chain branching distribution is narrow.
Long-chain branching in all polymers, including polyethylenes, changes their melt Theological behavior, typically making their flow more non-Newtonian over a large range of shear. Broader molecular weight distribution (MWD), without any branching, also increases non-Newtonian behavior. In LDPE, long-chain branching and broad MWD combine to provide considerable non-Newtonian behavior. Here however, long-chain branching per se, in addition to the nature of the polymerization, causes a broadening of the MWD, so that long-chain branching, broad MWD, and non-Newtonian rheology are inextricably intertwined. In certain metallocene polyethylenes, it has been found possible to have a small amount of long-chain branching which, because of its uniform positioning along the polymer chains and from chain to chain, allows the MWD to remain narrow, yet provides considerable non-Newtonian behavior. The narrow MWD provides, in general, superior properties, and the non-Newtonian behavior provides, in general, superior processability. Such long-chain branching is not necessarily present in metallocene polyethylenes however, and such metallocene polyethylenes are generally more Newtonian in their rheological behavior.
U.S. Pat. No. 5,272,236 (Lai et al.) and its continuation-in-part U.S. Pat. No. 5,278,272 (also Lai et al.) disclose metallocene polyethylene homopolymers and copolymers which have a small amount of controlled long-chain branching which causes advantageous rheology, but without broadening MWD. The amount of branching is from 0.1 to 3 long-chain branches (lcbs) per 1000 chain carbon atoms. These polyethylenes with this deliberate, small amount of long-chain branching are referred to, in a logical `tour de force` as `substantially linear`. (The first of these two patents, allows for `unsubstituted` non-branched polymer as being within the definition of substantially linear, i.e., it also includes from 0 to 0.1 lcbs per 1000 chain carbon atoms). In these two patents, long-chain branching is described as being due to carbon side chains of `at least 6 carbon atoms`. The long-chain branching is produced by certain polymerization conditions, and not by any added polymerizable species.
Short-chain branching, also uniformly positioned along the chain, can be introduced by C3-C20 alpha-olefins as well as certain acetylenically unsaturated and diolefin monomers. In practice the comonomer in metallocene polymers is typically butene or hexene, as in Exxon EXACT (TM) resins and octene in Dow AFFINITY(TM) and ENGAGE (TM) resins. Also used are propylene and norbornadiene in more elastomeric versions of metallocene polyolefin based resins. The amount of comonomer may be up to at least 30 mole percent, and these levels change the density of the polyethylenes in a comparable way to the change in density from conventional-HDPE through conventional-LLDPE, to the so-called very low density polyethylene with high comonomer content, (conventional-VLDPE) and finally to elastomers, usually with very high comonomer content.
In the above two patents, octene is typically the comonomer. Though octene will produce a side chain of 6 carbon atoms, and is introduced at a mole percent level of at least 5 percent, it is apparently not counted as a long-chain branch, despite their definition of long-chain branch. Long-chain branching appears only to refer to polymerization-produced and not comonomer-introduced branches.
These so-called `substantially linear` metallocene polyethylenes as in the above two patents, which have been grafted with acid comonomers such as maleic anhydride are the subject of U.S. Pat. No. 5,346,963 (Hughes et al.) After grafting the advantageous MWD and non-Newtonian rheology and good melt flow of the before-grafting metallocene resins are disclosed as remaining intact, unlike grafting on conventional polyethylenes, which, according to the above patent, can cause poor rheology. The grafted resins are disclosed as being advantageous in compatibilizing various thermoplastics including olefin and non-olefin polymers, as well as in compatibilizing filler and matrix in particulate-filled resins. Blends which include blends with (non-grafted) conventional polyethylenes and LDPE, non-grafted substantially linear polyethylenes, as well as a vast range including many ethylene copolymers such as EVOH, EVA and many non-ethylene polymers, are disclosed as being extrudable into shaped articles. The grafted substantially linear polyethylenes are disclosed as being useful when made into a film `comprising up to 100% of the graft polymer` These films exhibit desirable adhesive properties and are useful as tie layers in tying, for instance, polyethylene to EVOH. The films described and tested are prepared from 100% of the grafted substantially linear resins. Heat-seal tests described show such films seal better to polypropylene, polyamide and polycarbonate, but seal to EVOH about equally well as grafted conventional-linear polyethylene does.
There remains a continuing need for adhesives which possess superior properties to prior art conventional ethylene copolymer based adhesives or adhesive blends.